forked from syndicate-lang/preserves
988 lines
40 KiB
Markdown
988 lines
40 KiB
Markdown
---
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no_site_title: true
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title: "Preserves: an Expressive Data Language"
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---
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Tony Garnock-Jones <tonyg@leastfixedpoint.com>
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August 2019. Version 0.0.6.
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[sexp.txt]: http://people.csail.mit.edu/rivest/Sexp.txt
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[spki]: http://world.std.com/~cme/html/spki.html
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[varint]: https://developers.google.com/protocol-buffers/docs/encoding#varints
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[erlang-map]: http://erlang.org/doc/reference_manual/data_types.html#map
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[abnf]: https://tools.ietf.org/html/rfc7405
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This document proposes a data model and serialization format called
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*Preserves*.
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Preserves supports *records* with user-defined *labels*. This relieves
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the confusion caused by encoding records as dictionaries, seen in most
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data languages in use on the web. It also allows Preserves to easily
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represent the *labelled sums of products* as seen in many functional
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programming languages.
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Preserves also supports the usual suite of atomic and compound data
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types, in particular including *binary* data as a distinct type from
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text strings. Its *annotations* allow separation of data from metadata
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such as [comments](conventions.html#comments), trace information, and
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provenance information.
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Finally, Preserves defines precisely how to *compare* two values.
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Comparison is based on the data model, not on syntax or on data
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structures of any particular implementation language.
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## Starting with Semantics
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Taking inspiration from functional programming, we start with a
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definition of the *values* that we want to work with and give them
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meaning independent of their syntax.
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Our `Value`s fall into two broad categories: *atomic* and *compound*
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data.
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Value = Atom
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| Compound
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Atom = Boolean
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| Float
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| Double
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| SignedInteger
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| String
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| ByteString
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| Symbol
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Compound = Record
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| Sequence
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| Set
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| Dictionary
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**Total order.**<a name="total-order"></a> As we go, we will
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incrementally specify a total order over `Value`s. Two values of the
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same kind are compared using kind-specific rules. The ordering among
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values of different kinds is essentially arbitrary, but having a total
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order is convenient for many tasks, so we define it as
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follows:[^ordering-by-syntax]
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(Values) Atom < Compound
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(Compounds) Record < Sequence < Set < Dictionary
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(Atoms) Boolean < Float < Double < SignedInteger
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< String < ByteString < Symbol
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[^ordering-by-syntax]: The observant reader may note that the
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ordering here is the same as that implied by the tagging scheme
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used in the concrete binary syntax for `Value`s.
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**Equivalence.**<a name="equivalence"></a> Two `Value`s are equal if
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neither is less than the other according to the total order.
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### Signed integers.
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A `SignedInteger` is a signed integer of arbitrary width.
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`SignedInteger`s are compared as mathematical integers.
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### Unicode strings.
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A `String` is a sequence of Unicode
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[code-point](http://www.unicode.org/glossary/#code_point)s. `String`s
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are compared lexicographically, code-point by
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code-point.[^utf8-is-awesome]
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[^utf8-is-awesome]: Happily, the design of UTF-8 is such that this
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gives the same result as a lexicographic byte-by-byte comparison
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of the UTF-8 encoding of a string!
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### Binary data.
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A `ByteString` is a sequence of octets. `ByteString`s are compared
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lexicographically.
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### Symbols.
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Programming languages like Lisp and Prolog frequently use string-like
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values called *symbols*. Here, a `Symbol` is, like a `String`, a
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sequence of Unicode code-points representing an identifier of some
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kind. `Symbol`s are also compared lexicographically by code-point.
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### Booleans.
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There are two `Boolean`s, “false” and “true”. The “false” value is
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less-than the “true” value.
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### IEEE floating-point values.
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`Float`s and `Double`s are single- and double-precision IEEE 754
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floating-point values, respectively. `Float`s, `Double`s and
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`SignedInteger`s are disjoint; by the rules [above](#total-order),
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every `Float` is less than every `Double`, and every `SignedInteger`
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is greater than both. Two `Float`s or two `Double`s are to be ordered
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by the `totalOrder` predicate defined in section 5.10 of
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[IEEE Std 754-2008](https://dx.doi.org/10.1109/IEEESTD.2008.4610935).
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### Records.
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A `Record` is a *labelled* tuple of `Value`s, the record's *fields*. A
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label can be any `Value`, but is usually a `Symbol`.[^extensibility]
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[^iri-labels] `Record`s are compared lexicographically: first by
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label, then by field sequence.
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[^extensibility]: The [Racket](https://racket-lang.org/) programming
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language defines
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“[prefab](http://docs.racket-lang.org/guide/define-struct.html#(part._prefab-struct))”
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structure types, which map well to our `Record`s. Racket supports
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record extensibility by encoding record supertypes into record
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labels as specially-formatted lists.
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[^iri-labels]: It is occasionally (but seldom) necessary to
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interpret such `Symbol` labels as UTF-8 encoded IRIs. Where a
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label can be read as a relative IRI, it is notionally interpreted
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with respect to the IRI
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`urn:uuid:6bf094a6-20f1-4887-ada7-46834a9b5b34`; where a label can
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be read as an absolute IRI, it stands for that IRI; and otherwise,
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it cannot be read as an IRI at all, and so the label simply stands
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for itself—for its own `Value`.
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### Sequences.
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A `Sequence` is a sequence of `Value`s. `Sequence`s are compared
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lexicographically.
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### Sets.
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A `Set` is an unordered finite set of `Value`s. It contains no
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duplicate values, following the [equivalence relation](#equivalence)
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induced by the total order on `Value`s. Two `Set`s are compared by
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sorting their elements ascending using the [total order](#total-order)
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and comparing the resulting `Sequence`s.
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### Dictionaries.
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A `Dictionary` is an unordered finite collection of pairs of `Value`s.
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Each pair comprises a *key* and a *value*. Keys in a `Dictionary` are
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pairwise distinct. Instances of `Dictionary` are compared by
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lexicographic comparison of the sequences resulting from ordering each
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`Dictionary`'s pairs in ascending order by key.
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## Textual Syntax
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Now we have discussed `Value`s and their meanings, we may turn to
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techniques for *representing* `Value`s for communication or storage.
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In this section, we use [case-sensitive ABNF][abnf] to define a
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textual syntax that is easy for people to read and
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write.[^json-superset] Most of the examples in this document are
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written using this syntax. In the following section, we will define an
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equivalent compact machine-readable syntax.
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[^json-superset]: The grammar of the textual syntax is a superset of
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JSON, with the slightly unusual feature that `true`, `false`, and
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`null` are all read as `Symbol`s, and that `SignedInteger`s are
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never read as `Double`s.
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### Character set.
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[ABNF][abnf] allows easy definition of US-ASCII-based languages.
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However, Preserves is a Unicode-based language. Therefore, we
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reinterpret ABNF as a grammar for recognising sequences of Unicode
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code points.
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Textual syntax for a `Value` *SHOULD* be encoded using UTF-8 where
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possible.
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### Whitespace.
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Whitespace is defined as any number of spaces, tabs, carriage returns,
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line feeds, or commas.
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ws = *(%x20 / %x09 / newline / ",")
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newline = CR / LF
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### Grammar.
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Standalone documents may have trailing whitespace.
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Document = Value ws
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Any `Value` may be preceded by whitespace.
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Value = ws (Record / Collection / Atom / Compact)
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Collection = Sequence / Dictionary / Set
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Atom = Boolean / Float / Double / SignedInteger /
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String / ByteString / Symbol
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Each `Record` is an angle-bracket enclosed grouping of its
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label-`Value` followed by its field-`Value`s.
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Record = "<" Value *Value ws ">"
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`Sequence`s are enclosed in square brackets. `Dictionary` values are
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curly-brace-enclosed colon-separated pairs of values. `Set`s are
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written either as one or more values enclosed in curly braces, or zero
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or more values enclosed by the tokens `#set{` and
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`}`.[^printing-collections] It is an error for a set to contain
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duplicate elements or for a dictionary to contain duplicate keys.
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Sequence = "[" *Value ws "]"
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Dictionary = "{" *(Value ws ":" Value) ws "}"
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Set = %s"#set{" *Value ws "}" / "{" 1*Value ws "}"
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[^printing-collections]: **Implementation note.** When implementing
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printing of `Value`s using the textual syntax, consider supporting
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(a) optional pretty-printing with indentation, (b) optional
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JSON-compatible print mode for that subset of `Value` that is
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compatible with JSON, and (c) optional submodes for no commas,
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commas separating, and commas terminating elements or key/value
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pairs within a collection.
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`Boolean`s are the simple literal strings `#true` and `#false`.
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Boolean = %s"#true" / %s"#false"
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Numeric data follow the
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[JSON grammar](https://tools.ietf.org/html/rfc8259#section-6), with
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the addition of a trailing “f” distinguishing `Float` from `Double`
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values. `Float`s and `Double`s always have either a fractional part or
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an exponent part, where `SignedInteger`s never have
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either.[^reading-and-writing-floats-accurately]
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[^arbitrary-precision-signedinteger]
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Float = flt %i"f"
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Double = flt
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SignedInteger = int
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digit1-9 = %x31-39
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nat = %x30 / ( digit1-9 *DIGIT )
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int = ["-"] nat
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frac = "." 1*DIGIT
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exp = %i"e" ["-"/"+"] 1*DIGIT
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flt = int (frac exp / frac / exp)
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[^reading-and-writing-floats-accurately]: **Implementation note.**
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Your language's standard library likely has a good routine for
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converting between decimal notation and IEEE 754 floating-point.
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However, if not, or if you are interested in the challenges of
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accurately reading and writing floating point numbers, see the
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excellent matched pair of 1990 papers by Clinger and Steele &
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White, and a recent follow-up by Jaffer:
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Clinger, William D. ‘How to Read Floating Point Numbers
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Accurately’. In Proc. PLDI. White Plains, New York, 1990.
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<https://doi.org/10.1145/93542.93557>.
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Steele, Guy L., Jr., and Jon L. White. ‘How to Print
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Floating-Point Numbers Accurately’. In Proc. PLDI. White Plains,
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New York, 1990. <https://doi.org/10.1145/93542.93559>.
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Jaffer, Aubrey. ‘Easy Accurate Reading and Writing of
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Floating-Point Numbers’. ArXiv:1310.8121 [Cs], 27 October 2013.
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<http://arxiv.org/abs/1310.8121>.
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[^arbitrary-precision-signedinteger]: **Implementation note.** Be
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aware when implementing reading and writing of `SignedInteger`s
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that the data model *requires* arbitrary-precision integers. Your
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I/O routines must not truncate precision either when reading or
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writing a `SignedInteger`.
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`String`s are,
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[as in JSON](https://tools.ietf.org/html/rfc8259#section-7), possibly
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escaped text surrounded by double quotes. The escaping rules are the
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same as for JSON.[^string-json-correspondence] [^escaping-surrogate-pairs]
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String = %x22 *char %x22
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char = unescaped / %x7C / escape (escaped / %x22 / %s"u" 4HEXDIG)
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unescaped = %x20-21 / %x23-5B / %x5D-7B / %x7D-10FFFF
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escape = %x5C ; \
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escaped = ( %x5C / ; \ reverse solidus U+005C
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%x2F / ; / solidus U+002F
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%x62 / ; b backspace U+0008
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%x66 / ; f form feed U+000C
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%x6E / ; n line feed U+000A
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%x72 / ; r carriage return U+000D
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%x74 ) ; t tab U+0009
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[^string-json-correspondence]: The grammar for `String` has the same
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effect as the
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[JSON](https://tools.ietf.org/html/rfc8259#section-7) grammar for
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`string`. Some auxiliary definitions (e.g. `escaped`) are lifted
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largely unmodified from the text of RFC 8259.
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[^escaping-surrogate-pairs]: In particular, note JSON's rules around
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the use of surrogate pairs for code points not in the Basic
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Multilingual Plane. We encourage implementations to avoid escaping
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such characters when producing output, and instead to rely on the
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UTF-8 encoding of the entire document to handle them correctly.
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A `ByteString` may be written in any of three different forms.
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The first is similar to a `String`, but prepended with a hash sign
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`#`. In addition, only Unicode code points overlapping with printable
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7-bit ASCII are permitted unescaped inside such a `ByteString`; other
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byte values must be escaped by prepending a two-digit hexadecimal
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value with `\x`.
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ByteString = "#" %x22 *binchar %x22
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binchar = binunescaped / escape (escaped / %x22 / %s"x" 2HEXDIG)
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binunescaped = %x20-21 / %x23-5B / %x5D-7E
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The second is as a sequence of pairs of hexadecimal digits interleaved
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with whitespace and surrounded by `#hex{` and `}`.
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ByteString =/ %s"#hex{" *(ws / 2HEXDIG) ws "}"
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The third is as a sequence of
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[Base64](https://tools.ietf.org/html/rfc4648) characters, interleaved
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with whitespace and surrounded by `#base64{` and `}`. Plain and
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URL-safe Base64 characters are allowed.
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ByteString =/ %s"#base64{" *(ws / base64char) ws "}" /
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base64char = %x41-5A / %x61-7A / %x30-39 / "+" / "/" / "-" / "_" / "="
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A `Symbol` may be written in a “bare” form[^cf-sexp-token] so long as
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it conforms to certain restrictions on the characters appearing in the
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symbol. Alternatively, it may be written in a quoted form. The quoted
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form is much the same as the syntax for `String`s, including embedded
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escape syntax, except using a bar or pipe character (`|`) instead of a
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double quote mark.
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Symbol = symstart *symcont / "|" *symchar "|"
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symstart = ALPHA / sympunct / symustart
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symcont = ALPHA / sympunct / symustart / symucont / DIGIT / "-"
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sympunct = "~" / "!" / "$" / "%" / "^" / "&" / "*" /
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"?" / "_" / "=" / "+" / "/" / "."
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symchar = unescaped / %x22 / escape (escaped / %x7C / %s"u" 4HEXDIG)
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symustart = <any code point greater than 127 whose Unicode
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category is Lu, Ll, Lt, Lm, Lo, Mn, Mc, Me,
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Pc, Po, Sc, Sm, Sk, So, or Co>
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symucont = <any code point greater than 127 whose Unicode
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category is Nd, Nl, No, or Pd>
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[^cf-sexp-token]: Compare with the [SPKI S-expression][sexp.txt]
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definition of “token representation”, and with the
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[R6RS definition of identifiers](http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-7.html#node_sec_4.2.4).
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Finally, any `Value` may be represented by escaping from the textual
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syntax to the [compact binary syntax](#compact-binary-syntax) by
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prefixing a `ByteString` containing the binary representation of the
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`Value` with `#value`.[^rationale-switch-to-binary]
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[^no-literal-binary-in-text] [^compact-value-annotations]
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Compact = %s"#value" ws ByteString
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[^rationale-switch-to-binary]: **Rationale.** The textual syntax
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cannot express every `Value`: specifically, it cannot express the
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several million floating-point NaNs, or the two floating-point
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Infinities. Since the compact binary format for `Value`s expresses
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each `Value` with precision, embedding binary `Value`s solves the
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problem.
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[^no-literal-binary-in-text]: Every text is ultimately physically
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stored as bytes; therefore, it might seem possible to escape to
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the raw binary form of compact binary encoding from within a
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pieces of textual syntax. However, while bytes must be involved in
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any *representation* of text, the text *itself* is logically a
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sequence of *code points* and is not *intrinsically* a binary
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structure at all. It would be incoherent to expect to be able to
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access the representation of the text from within the text itself.
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[^compact-value-annotations]: Any text-syntax annotations preceding
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the `#value` are prepended to any binary-syntax annotations
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yielded by decoding the `ByteString`.
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### Annotations.
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**Syntax.** When written down, a `Value` may have an associated
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sequence of *annotations* carrying “out-of-band” contextual metadata
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about the value. Each annotation is, in turn, a `Value`, and may
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itself have annotations.
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Value =/ ws "@" Value Value
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Each annotation is preceded by `@`; the underlying annotated value
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follows its annotations. Here we extend only the syntactic nonterminal
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named “`Value`” without altering the semantic class of `Value`s.
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**Equivalence.** Annotations appear within syntax denoting a `Value`;
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however, the annotations are not part of the denoted value. They are
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only part of the syntax. Annotations do not play a part in
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equivalences and orderings of `Value`s.
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Reflective tools such as debuggers, user interfaces, and message
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routers and relays---tools which process `Value`s generically---may
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use annotated inputs to tailor their operation, or may insert
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annotations in their outputs. By contrast, in ordinary programs, as a
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rule of thumb, the presence, absence or content of an annotation
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should not change the control flow or output of the program.
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Annotations are data *describing* `Value`s, and are not in the domain
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of any specific application of `Value`s. That is, an annotation will
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almost never cause a non-reflective program to do anything observably
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different.
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## Compact Binary Syntax
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A `Repr` is a binary-syntax encoding, or representation, of either
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- a `Value`,
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- a “placeholder” for a `Value`, or
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- an annotation on a `Repr`.
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Each `Repr` comprises one or more bytes describing the kind of
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represented information and the length of the representation, followed
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by the encoded details.
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For a value `v`, we write `[[v]]` for the `Repr` of v.
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### Type and Length representation.
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Each `Repr` takes one of three possible forms:
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- (A) type-specific form, used for simple values such as `Boolean`s
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or `Float`s, for placeholders, and for introducing annotations.
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- (B) a variable-length form with length specified up-front, used for
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compound and variable-length atomic data structures when their
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sizes are known at the time serialization begins.
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- (C) a variable-length streaming form with unknown or unpredictable
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length, used in cases when serialization begins before the number
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of elements or bytes in the corresponding `Value` is known.
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Applications may choose between formats B and C depending on their
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needs at serialization time.
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#### The lead byte.
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Every `Repr` starts with a *lead byte*, constructed by
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`leadbyte(t,n,m)`, where `t`,`n`∈{0,1,2,3} and 0≤`m`<16:
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leadbyte(t,n,m) = [t*64 + n*16 + m]
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The arguments `t`, `n` and `m` describe the rest of the
|
||
representation.[^some-encodings-unused]
|
||
|
||
[^some-encodings-unused]: Some encodings are unused. All such
|
||
encodings are reserved for future versions of this specification.
|
||
|
||
| `t` | `n` | `m` | Meaning |
|
||
| --- | --- | --- | ------- |
|
||
| 0 | 0 | 0–3 | (format A) An `Atom` with fixed-length binary representation |
|
||
| 0 | 0 | 4 | (format C) Stream end |
|
||
| 0 | 0 | 5 | (format A) Annotation |
|
||
| 0 | 1 | | (format A) Placeholder for an application-specific `Value` |
|
||
| 0 | 2 | | (format C) Stream start |
|
||
| 0 | 3 | | (format A) Certain small `SignedInteger`s |
|
||
| 1 | | | (format B) An `Atom` with variable-length binary representation |
|
||
| 2 | | | (format B) A `Compound` with variable-length representation |
|
||
|
||
#### Encoding data of type-specific length (format A).
|
||
|
||
Each type of data defines its own rules for this format.
|
||
|
||
#### Encoding data of known length (format B).
|
||
|
||
Format B is used where the length `l` of the `Value` to be encoded is
|
||
known when serialization begins. Format B `Repr`s use `m` in
|
||
`leadbyte` to encode `l`. The length counts *bytes* for atomic
|
||
`Value`s, but counts *contained values* for compound `Value`s.
|
||
|
||
- A length `l` between 0 and 14 is represented using `leadbyte` with
|
||
`m=l`.
|
||
- A length of 15 or greater is represented by `m=15` and additional
|
||
bytes describing the length following the lead byte.
|
||
|
||
The function `header(t,n,m)` yields an appropriate sequence of bytes
|
||
describing a `Repr`'s type and length when `t`, `n` and `m` are
|
||
appropriate non-negative integers:
|
||
|
||
header(t,n,m) = leadbyte(t,n,m) when m < 15
|
||
or leadbyte(t,n,15) ++ varint(m) otherwise
|
||
|
||
The additional length bytes are formatted as
|
||
[base 128 varints][varint]. We write `varint(m)` for the
|
||
varint-encoding of `m`. Quoting the [Google Protocol Buffers][varint]
|
||
definition,
|
||
|
||
> Each byte in a varint, except the last byte, has the most
|
||
> significant bit (msb) set – this indicates that there are further
|
||
> bytes to come. The lower 7 bits of each byte are used to store the
|
||
> two's complement representation of the number in groups of 7 bits,
|
||
> least significant group first.
|
||
|
||
The following table illustrates varint-encoding.
|
||
|
||
| Number, `m` | `m` in binary, grouped into 7-bit chunks | `varint(m)` bytes |
|
||
| ------ | ------------------- | ------------ |
|
||
| 15 | `0001111` | 15 |
|
||
| 300 | `0000010 0101100` | 172 2 |
|
||
| 1000000000 | `0000011 1011100 1101011 0010100 0000000` | 128 148 235 220 3 |
|
||
|
||
#### Streaming data of unknown length (format C).
|
||
|
||
A `Repr` where the length of the `Value` to be encoded is variable and
|
||
not known at the time serialization of the `Value` starts is encoded
|
||
by a single Stream Start (“open”) byte, followed by zero or more
|
||
*chunks*, followed by a matching Stream End (“close”) byte:
|
||
|
||
open(t,n) = leadbyte(0,2, t*4 + n) = [0x20 + t*4 + n]
|
||
close() = leadbyte(0,0, 4) = [0x04]
|
||
|
||
For a format C `Repr` of an atomic `Value`, each chunk is to be a
|
||
format B `Repr` of a `ByteString`, no matter the type of the overall
|
||
`Value`. Annotations are not allowed on these individual chunks.
|
||
|
||
For a format C `Repr` of a compound `Value`, each chunk is to be a
|
||
single `Repr`, which may itself be annotated.
|
||
|
||
Each chunk within a format C `Repr` *MUST* have non-zero length.
|
||
Software that decodes `Repr`s *MUST* reject `Repr`s that include
|
||
zero-length chunks.
|
||
|
||
### Records.
|
||
|
||
Format B (known length):
|
||
|
||
[[ <L F_1...F_m> ]] = header(2,0,m+1) ++ [[L]] ++ [[F_1]] ++...++ [[F_m]]
|
||
|
||
For `m` fields, `m+1` is supplied to `header`, to account for the
|
||
encoding of the record label.
|
||
|
||
Format C (streaming):
|
||
|
||
[[ <L F_1...F_m> ]] = open(2,0) ++ [[L]] ++ [[F_1]] ++...++ [[F_m]] ++ close()
|
||
|
||
Applications *SHOULD* prefer the known-length format for encoding
|
||
`Record`s.
|
||
|
||
### Placeholders.
|
||
|
||
Applications may define an interpretation for numbered *placeholders*
|
||
in the binary syntax, mapping each *placeholder number* `n` to a
|
||
specific `Value`. For example, a placeholder number may be assigned
|
||
for a frequently-used `Record` label.
|
||
|
||
A `Value` `v` for which placeholder number `n` has been assigned may
|
||
be tersely encoded as
|
||
|
||
[[v]] = header(0,1,n) when n is a placeholder number for v
|
||
|
||
**Examples.** For example, a protocol may choose to assign placeholder
|
||
number 4 to the symbol `void`, making
|
||
|
||
[[void]] = header(0,1,4) = [0x14]
|
||
[[<void>]] = header(2,0,1) ++ [[void]] = [0x81, 0x14]
|
||
|
||
or it may map symbol `person` to placeholder number 102, making
|
||
|
||
[[person]] = header(0,1,102) = [0x1F, 0x66]
|
||
|
||
and so
|
||
|
||
[[<person "Dr" "Elizabeth" "Blackwell">]]
|
||
= header(2,0,4) ++ [[person]] ++ [["Dr"]] ++ [["Elizabeth"]] ++ [["Blackwell"]]
|
||
= [0x84, 0x1F, 0x66] ++ [["Dr"]] ++ [["Elizabeth"]] ++ [["Blackwell"]]
|
||
|
||
for format B, or
|
||
|
||
open(2,0) ++ [[person]] ++ [["Dr"]] ++ [["Elizabeth"]] ++ [["Blackwell"]] ++ close()
|
||
= [0x28, 0x1F, 0x66] ++ [["Dr"]] ++ [["Elizabeth"]] ++ [["Blackwell"]] ++ [0x04]
|
||
|
||
for format C.
|
||
|
||
### Sequences, Sets and Dictionaries.
|
||
|
||
Format B (known length):
|
||
|
||
[[ [X_1...X_m] ]] = header(2,1,m) ++ [[X_1]] ++...++ [[X_m]]
|
||
[[ #set{X_1...X_m} ]] = header(2,2,m) ++ [[X_1]] ++...++ [[X_m]]
|
||
[[ {K_1:V_1...K_m:V_m} ]] = header(2,3,m*2) ++ [[K_1]] ++ [[V_1]] ++...
|
||
++ [[K_m]] ++ [[V_m]]
|
||
|
||
Note that `m*2` is given to `header` for a `Dictionary`, since there
|
||
are two `Value`s in each key-value pair.
|
||
|
||
Format C (streaming):
|
||
|
||
[[ [X_1...X_m] ]] = open(2,1) ++ [[X_1]] ++...++ [[X_m]] ++ close()
|
||
[[ #set{E_1...E_m} ]] = open(2,2) ++ [[E_1]] ++...++ [[E_m]] ++ close()
|
||
[[ {K_1:V_1...K_m:V_m} ]] = open(2,3) ++ [[K_1]] ++ [[V_1]] ++...
|
||
++ [[K_m]] ++ [[V_m]] ++ close()
|
||
|
||
Applications may use whichever format suits their needs on a
|
||
case-by-case basis.
|
||
|
||
There is *no* ordering requirement on the `E_i` elements or
|
||
`K_i`/`V_i` pairs.[^no-sorting-rationale] They may appear in any
|
||
order. However, the `E_i` and `K_i` *MUST* be pairwise distinct.
|
||
|
||
[^no-sorting-rationale]: In the BitTorrent encoding format,
|
||
[bencoding](http://www.bittorrent.org/beps/bep_0003.html#bencoding),
|
||
dictionary key/value pairs must be sorted by key. This is a
|
||
necessary step for ensuring serialization of `Value`s is
|
||
canonical. We do not require that key/value pairs (or set
|
||
elements) be in sorted order for serialized `Value`s, because (a)
|
||
where canonicalization is used for cryptographic signatures, it is
|
||
more reliable to simply retain the exact binary form of the signed
|
||
document than to depend on canonical de- and re-serialization, and
|
||
(b) sorting keys or elements makes no sense in streaming
|
||
serialization formats.
|
||
|
||
However, a quality implementation may wish to offer the programmer
|
||
the option of serializing with set elements and dictionary keys in
|
||
sorted order.
|
||
|
||
### SignedIntegers.
|
||
|
||
Format B/A (known length/fixed-size):
|
||
|
||
[[ x ]] when x ∈ SignedInteger = header(1,0,m) ++ intbytes(x) if x<-3 ∨ 13≤x
|
||
header(0,3,x+16) if -3≤x<0
|
||
header(0,3,x) if 0≤x<13
|
||
|
||
Integers in the range [-3,12] are compactly represented using format A
|
||
because they are so frequently used. Other integers are represented
|
||
using format B.
|
||
|
||
Format C *MUST NOT* be used for `SignedInteger`s.
|
||
|
||
The function `intbytes(x)` gives the big-endian two's-complement
|
||
binary representation of `x`, taking exactly as many whole bytes as
|
||
needed to unambiguously identify the value and its sign, and `m =
|
||
|intbytes(x)|`. The most-significant bit in the first byte in
|
||
`intbytes(x)` <!-- for `x`≠0 --> is the sign bit.[^zero-intbytes]
|
||
|
||
[^zero-intbytes]: The value 0 needs zero bytes to identify the
|
||
value, so `intbytes(0)` is the empty byte string. Non-zero values
|
||
need at least one byte.
|
||
|
||
For example,
|
||
|
||
[[ -257 ]] = 42 FE FF [[ -3 ]] = 3D [[ 128 ]] = 42 00 80
|
||
[[ -256 ]] = 42 FF 00 [[ -2 ]] = 3E [[ 255 ]] = 42 00 FF
|
||
[[ -255 ]] = 42 FF 01 [[ -1 ]] = 3F [[ 256 ]] = 42 01 00
|
||
[[ -254 ]] = 42 FF 02 [[ 0 ]] = 30 [[ 32767 ]] = 42 7F FF
|
||
[[ -129 ]] = 42 FF 7F [[ 1 ]] = 31 [[ 32768 ]] = 43 00 80 00
|
||
[[ -128 ]] = 41 80 [[ 12 ]] = 3C [[ 65535 ]] = 43 00 FF FF
|
||
[[ -127 ]] = 41 81 [[ 13 ]] = 41 0D [[ 65536 ]] = 43 01 00 00
|
||
[[ -4 ]] = 41 FC [[ 127 ]] = 41 7F [[ 131072 ]] = 43 02 00 00
|
||
|
||
### Strings, ByteStrings and Symbols.
|
||
|
||
Syntax for these three types varies only in the value of `n` supplied
|
||
to `header` and `open`. In each case, the payload following the header
|
||
is a binary sequence; for `String` and `Symbol`, it is a UTF-8
|
||
encoding of the `Value`'s code points, while for `ByteString` it is
|
||
the raw data contained within the `Value` unmodified.
|
||
|
||
Format B (known length):
|
||
|
||
[[ S ]] = header(1,n,m) ++ encode(S)
|
||
where m = |encode(S)|
|
||
and (n,encode(S)) = (1,utf8(S)) if S ∈ String
|
||
(2,S) if S ∈ ByteString
|
||
(3,utf8(S)) if S ∈ Symbol
|
||
|
||
To stream a `String`, `ByteString` or `Symbol`, emit `open(1,n)` and
|
||
then a sequence of zero or more format B chunks, followed by
|
||
`close()`. Every chunk must be a `ByteString`, and no chunk may be
|
||
annotated.
|
||
|
||
While the overall content of a streamed `String` or `Symbol` must be
|
||
valid UTF-8, individual chunks do not have to conform to UTF-8.
|
||
|
||
### Fixed-length Atoms.
|
||
|
||
Fixed-length atoms all use format A, and do not have a length
|
||
representation. They repurpose the bits that format B `Repr`s use to
|
||
specify lengths. Applications *MUST NOT* use format C with `open(0,n)`
|
||
for any `n`.
|
||
|
||
#### Booleans.
|
||
|
||
[[ #false ]] = header(0,0,0) = [0x00]
|
||
[[ #true ]] = header(0,0,1) = [0x01]
|
||
|
||
#### Floats and Doubles.
|
||
|
||
[[ F ]] when F ∈ Float = header(0,0,2) ++ binary32(F)
|
||
[[ D ]] when D ∈ Double = header(0,0,3) ++ binary64(D)
|
||
|
||
The functions `binary32(F)` and `binary64(D)` yield big-endian 4- and
|
||
8-byte IEEE 754 binary representations of `F` and `D`, respectively.
|
||
|
||
### Annotations.
|
||
|
||
To annotate a `Repr` `r` with some `Value` `v`, prepend `r` with
|
||
`[0x05] ++ [[v]]`.
|
||
|
||
For example, the `Repr` corresponding to textual syntax `@a@b[]`,
|
||
i.e. an empty sequence annotated with two symbols, `a` and `b`, is
|
||
|
||
[[ @a @b [] ]]
|
||
= [0x05] ++ [[a]] ++ [0x05] ++ [[b]] ++ [[ [] ]]
|
||
= [0x05, 0x71, 0x61, 0x05, 0x71, 0x62, 0x90]
|
||
|
||
## Examples
|
||
|
||
### Simple examples.
|
||
|
||
<!-- TODO: Give some examples of large and small Preserves, perhaps -->
|
||
<!-- translated from various JSON blobs floating around the internet. -->
|
||
|
||
For the following examples, imagine an application that maps
|
||
placeholder number 0 to symbol `discard`, 1 to `capture`, and 2 to
|
||
`observe`.
|
||
|
||
| Value | Encoded byte sequence |
|
||
|---------------------------------------------------|-------------------------------------------------------------------------------------|
|
||
| `<capture <discard>>` | 82 11 81 10 |
|
||
| `<observe <speak <discard> <capture <discard>>>>` | 82 12 83 75 's' 'p' 'e' 'a' 'k' 81 10 82 11 81 11 |
|
||
| `[1 2 3 4]` (format B) | 94 31 32 33 34 |
|
||
| `[1 2 3 4]` (format C) | 29 31 32 33 34 04 |
|
||
| `[-2 -1 0 1]` | 94 3E 3F 30 31 |
|
||
| `"hello"` (format B) | 55 'h' 'e' 'l' 'l' 'o' |
|
||
| `"hello"` (format C, 2 chunks) | 25 62 'h' 'e' 63 'l' 'l' 'o' 35 |
|
||
| `"hello"` (format C, 5 chunks) | 25 61 'h' 61 'e' 61 'l' 61 'l' 61 'o' 35 |
|
||
| `["hello" there #"world" [] #set{} #true #false]` | 97 55 'h' 'e' 'l' 'l' 'o' 75 't' 'h' 'e' 'r' 'e' 65 'w' 'o' 'r' 'l' 'd' 90 A0 01 00 |
|
||
| `-257` | 42 FE FF |
|
||
| `-1` | 3F |
|
||
| `0` | 30 |
|
||
| `1` | 31 |
|
||
| `255` | 42 00 FF |
|
||
| `1.0f` | 02 3F 80 00 00 |
|
||
| `1.0` | 03 3F F0 00 00 00 00 00 00 |
|
||
| `-1.202e300` | 03 FE 3C B7 B7 59 BF 04 26 |
|
||
|
||
The next example uses a non-`Symbol` label for a record.[^extensibility2] The `Record`
|
||
|
||
<[titled person 2 thing 1] 101 "Blackwell" <date 1821 2 3> "Dr">
|
||
|
||
encodes to
|
||
|
||
85 ;; Record, generic, 4+1
|
||
95 ;; Sequence, 5
|
||
76 74 69 74 6C 65 64 ;; Symbol, "titled"
|
||
76 70 65 72 73 6F 6E ;; Symbol, "person"
|
||
32 ;; SignedInteger, "2"
|
||
75 74 68 69 6E 67 ;; Symbol, "thing"
|
||
31 ;; SignedInteger, "1"
|
||
41 65 ;; SignedInteger, "101"
|
||
59 42 6C 61 63 6B 77 65 6C 6C ;; String, "Blackwell"
|
||
84 ;; Record, generic, 3+1
|
||
74 64 61 74 65 ;; Symbol, "date"
|
||
42 07 1D ;; SignedInteger, "1821"
|
||
32 ;; SignedInteger, "2"
|
||
33 ;; SignedInteger, "3"
|
||
52 44 72 ;; String, "Dr"
|
||
|
||
[^extensibility2]: It happens to line up with Racket's
|
||
representation of a record label for an inheritance hierarchy
|
||
where `titled` extends `person` extends `thing`:
|
||
|
||
(struct date (year month day) #:prefab)
|
||
(struct thing (id) #:prefab)
|
||
(struct person thing (name date-of-birth) #:prefab)
|
||
(struct titled person (title) #:prefab)
|
||
|
||
For more detail on Racket's representations of record labels, see
|
||
[the Racket documentation for `make-prefab-struct`](http://docs.racket-lang.org/reference/structutils.html#%28def._%28%28quote._~23~25kernel%29._make-prefab-struct%29%29).
|
||
|
||
---
|
||
|
||
### JSON examples.
|
||
|
||
The examples from
|
||
[RFC 8259](https://tools.ietf.org/html/rfc8259#section-13) read as
|
||
valid Preserves, though the JSON literals `true`, `false` and `null`
|
||
read as `Symbol`s. The first example:
|
||
|
||
{
|
||
"Image": {
|
||
"Width": 800,
|
||
"Height": 600,
|
||
"Title": "View from 15th Floor",
|
||
"Thumbnail": {
|
||
"Url": "http://www.example.com/image/481989943",
|
||
"Height": 125,
|
||
"Width": 100
|
||
},
|
||
"Animated" : false,
|
||
"IDs": [116, 943, 234, 38793]
|
||
}
|
||
}
|
||
|
||
encodes to binary as follows:
|
||
|
||
B2
|
||
55 "Image"
|
||
BC
|
||
55 "Width" 42 03 20
|
||
55 "Title" 5F 14 "View from 15th Floor"
|
||
58 "Animated" 75 "false"
|
||
56 "Height" 42 02 58
|
||
59 "Thumbnail"
|
||
B6
|
||
55 "Width" 41 64
|
||
53 "Url" 5F 26 "http://www.example.com/image/481989943"
|
||
56 "Height" 41 7D
|
||
53 "IDs" 94
|
||
41 74
|
||
42 03 AF
|
||
42 00 EA
|
||
43 00 97 89
|
||
|
||
and the second example:
|
||
|
||
[
|
||
{
|
||
"precision": "zip",
|
||
"Latitude": 37.7668,
|
||
"Longitude": -122.3959,
|
||
"Address": "",
|
||
"City": "SAN FRANCISCO",
|
||
"State": "CA",
|
||
"Zip": "94107",
|
||
"Country": "US"
|
||
},
|
||
{
|
||
"precision": "zip",
|
||
"Latitude": 37.371991,
|
||
"Longitude": -122.026020,
|
||
"Address": "",
|
||
"City": "SUNNYVALE",
|
||
"State": "CA",
|
||
"Zip": "94085",
|
||
"Country": "US"
|
||
}
|
||
]
|
||
|
||
encodes to binary as follows:
|
||
|
||
92
|
||
BF 10
|
||
59 "precision" 53 "zip"
|
||
58 "Latitude" 03 40 42 E2 26 80 9D 49 52
|
||
59 "Longitude" 03 C0 5E 99 56 6C F4 1F 21
|
||
57 "Address" 50
|
||
54 "City" 5D "SAN FRANCISCO"
|
||
55 "State" 52 "CA"
|
||
53 "Zip" 55 "94107"
|
||
57 "Country" 52 "US"
|
||
BF 10
|
||
59 "precision" 53 "zip"
|
||
58 "Latitude" 03 40 42 AF 9D 66 AD B4 03
|
||
59 "Longitude" 03 C0 5E 81 AA 4F CA 42 AF
|
||
57 "Address" 50
|
||
54 "City" 59 "SUNNYVALE"
|
||
55 "State" 52 "CA"
|
||
53 "Zip" 55 "94085"
|
||
57 "Country" 52 "US"
|
||
|
||
## Security Considerations
|
||
|
||
**Empty chunks.** Chunks of zero length are prohibited in streamed
|
||
(format C) `Repr`s. However, a malicious or broken encoder may include
|
||
them nonetheless. This opens up a possibility for denial-of-service:
|
||
an attacker may begin streaming a `String`, for example, sending an
|
||
endless sequence of zero length chunks, appearing to make progress but
|
||
not actually doing so. Implementations *MUST* reject zero length
|
||
chunks when decoding, and *MUST NOT* produce them when encoding.
|
||
|
||
**Whitespace.** Similarly, the textual format for `Value`s allows
|
||
arbitrary whitespace in many positions. In streaming transfer
|
||
situations, consider optional restrictions on the amount of
|
||
consecutive whitespace that may appear in a serialized `Value`.
|
||
|
||
**Annotations.** Also similarly, in modes where a `Value` is being
|
||
read while annotations are skipped, an endless sequence of annotations
|
||
may give an illusion of progress.
|
||
|
||
**Canonical form for cryptographic hashing and signing.** As
|
||
specified, neither the textual nor the compact binary encoding rules
|
||
for `Value`s force canonical serializations. Two serializations of the
|
||
same `Value` may yield different binary `Repr`s.
|
||
|
||
## Acknowledgements
|
||
|
||
The use of low-order bits of each lead byte for the length of short
|
||
values is inspired by a similar feature of [CBOR](http://cbor.io/).
|
||
|
||
The treatment of commas as whitespace in the text syntax is inspired
|
||
by the same feature of [EDN](https://github.com/edn-format/edn).
|
||
|
||
The text syntax for `Boolean`s, `Symbol`s, and `ByteString`s is
|
||
directly inspired by [Racket](https://racket-lang.org/)'s lexical
|
||
syntax.
|
||
|
||
## Appendix. Table of lead byte values
|
||
|
||
00 - False
|
||
01 - True
|
||
02 - Float
|
||
03 - Double
|
||
04 - End stream
|
||
05 - Annotation
|
||
(0x) RESERVED 06-0F
|
||
1x - Placeholder
|
||
2x - Start Stream
|
||
3x - Small integers 0..12,-3..-1
|
||
|
||
4x - SignedInteger
|
||
5x - String
|
||
6x - ByteString
|
||
7x - Symbol
|
||
|
||
8x - Record
|
||
9x - Sequence
|
||
Ax - Set
|
||
Bx - Dictionary
|
||
|
||
(Cx) RESERVED C0-CF
|
||
(Dx) RESERVED D0-DF
|
||
(Ex) RESERVED E0-EF
|
||
(Fx) RESERVED F0-FF
|
||
|
||
## Appendix. Bit fields within lead byte values
|
||
|
||
tt nn mmmm contents
|
||
---------- ---------
|
||
|
||
00 00 0000 False
|
||
00 00 0001 True
|
||
00 00 0010 Float, 32 bits big-endian binary
|
||
00 00 0011 Double, 64 bits big-endian binary
|
||
00 00 0100 End Stream (to match a previous Start Stream)
|
||
00 00 0101 Annotation; two more Reprs follow
|
||
|
||
00 01 mmmm Placeholder; m is the placeholder number
|
||
|
||
00 10 ttnn Start Stream <tt,nn>
|
||
When tt = 00 --> error
|
||
01 --> each chunk is a ByteString
|
||
10 --> each chunk is a single encoded Value
|
||
11 --> error (RESERVED)
|
||
|
||
00 11 xxxx Small integers 0..12,-3..-1
|
||
|
||
01 00 mmmm SignedInteger, big-endian binary
|
||
01 01 mmmm String, UTF-8 binary
|
||
01 10 mmmm ByteString
|
||
01 11 mmmm Symbol, UTF-8 binary
|
||
|
||
10 00 mmmm Record
|
||
10 01 mmmm Sequence
|
||
10 10 mmmm Set
|
||
10 11 mmmm Dictionary
|
||
|
||
11 nn mmmm error, RESERVED
|
||
|
||
Where `mmmm` appears, interpret it as an unsigned 4-bit number `m`. If
|
||
`m`<15, let `l`=`m`. Otherwise, `m`=15; let `l` be the result of
|
||
decoding the varint that follows.
|
||
|
||
Then, if `ttnn`=`0001`, `l` is the placeholder number; otherwise, `l`
|
||
is the length of the body that follows, counted in bytes for `tt`=`01`
|
||
and in `Repr`s for `tt`=`10`.
|
||
|
||
<!-- Heading to visually offset the footnotes from the main document: -->
|
||
## Notes
|